But if you should happen to meet with a negro—a tall negro with a bandaged head or a contused wound of the scalp and a swollen leg—you had better keep your eye on him. The leg which is swollen is probably the left.”
The inspector was thrilled; and so was I, for that matter. The thing was incredible; but yet I knew that Thorndyke’s amazing deductions were the products of perfectly orthodox scientific methods. Only I could form no sort of guess as to how they had been arrived at. A negro’s blood is no different from any other person’s, and certainly affords no clue to his height or the condition of his legs. I could make nothing of it: and as the dialogue and the inspector’s note-takings brought us to the little town hall in which the inquest was to be held, I dismissed the puzzle until such time as Thorndyke chose to solve it.
When we entered the town hall we found everything in readiness for the opening of the proceedings. The jury were already in their places and the coroner was just about to take his seat at the head of the long table. We accordingly slipped on to the two chairs that were found for us by the inspector, and the latter took his place behind the jury and facing us. Near to him Mr. Foxley and Miss Markham were seated, and evidently hailed our arrival with profound relief, each of them smiling us a silent greeting. A professional-looking man sitting next to Thorndyke I assumed to be the medical witness, and a rather good young man who sat apart with a police constable I identified as Robert Fletcher.
The evidence of the “common” witnesses, who deposed to the general facts, told us nothing that we did not already know, excepting that it was made clear that Fletcher had left his uncle’s house not later than seven o’clock and that thereafter until the following morning his whereabouts were known. The medical witness was cautious, and kept an uneasy eye on Thorndyke. The wound which caused the death of deceased might have been inflicted by himself or by some other person. He had originally given the probable time of death as six or seven o’clock on Wednesday evening. He now admitted reply to a question from Thorndyke that he had not taken the temperature of the body, and that the rigidity and other conditions were not absolutely inconsistent with a considerable later time of death. Death might even have occurred after midnight.
In spite of this admission, however, the sum of the evidence tended strongly to implicate Fletcher, and one or two questions from jurymen suggested a growing belief in his guilt. I had no doubt whatever that if the case had been put to the jury at this stage, a unanimous verdict of “wilful murder” would have been the result. But, as the medical witness returned to his seat, the coroner fixed an inquisitive eye on Thorndyke.
“You have not been summoned as a witness, Dr. Thorndyke,” said he, “but I understand that you have made certain investigations in this case. Are you able to throw any fresh light on the circumstances of the death of the deceased, Joseph Riggs?
“Yes,” Thorndyke replied. “I am in a position to give important and material evidence.”
Thereupon he was sworn, and the coroner, still watching him curiously, said: “I am informed that you have examined samples of the blood of deceased and the blood which was found in the hall of deceased’s house. Did you examine them, and if so, what was the object of the examination?”
“I examined both samples and also samples of the blood of Robert Fletcher. The object was to ascertain whether the blood on the hall floor was the blood of the deceased or of Robert Fletcher.”
The coroner glanced at the medical witness, and a faint smile appeared on the face of each.
“And did you,” the former asked in a slightly ironical tone, “form any opinion on the subject?”
“I ascertained definitely that the blood in the hall was neither that of the deceased nor that of Robert Fletcher.”
The coroner’s eyebrows went up, and once more he glanced significantly at the doctor.
“But,” he demanded incredulously, “is it possible to distinguish the blood of one person from that of another?”
“Usually it is not, but in certain exceptional cases it is. This happened to be an exceptional case.”
“In what respect?”
“It happened,” Thorndyke replied, “that the person whose blood was found in the hall suffered from the parasitic disease known as filariasis. His blood was infested with swarms of a minute worm named Filaria nocturna. I have here,” he continued, taking out of his research-case the two bottles and the three boxes, “thirty-six mounted specimens of this blood, and in every one of them one or more of the parasites is to be seen. I have also thirty-six mounted specimens each of the blood of the deceased, and the blood of Robert Fletcher. In not one of these specimens is a single parasite to be found. Moreover, I have examined Robert Fletcher and the body of the deceased, and can testify that no sign of filarial disease was to be discovered in either. Hence it is certain, that the blood found in the halt was not the blood of either of these two persons.”
The ironic smile had faded from the coroner’s face. He was evidently deeply impressed, and his manner was quite deferential as he asked: “Do these very remarkable observations of yours lead to any further inferences?”
“Yes,” replied Thorndyke. “They render it certain that this blood was shed no earlier than nine o’clock and probably nearer midnight.”
“Really!” the astonished coroner exclaimed. “Now, how is it possible to fix the time in that exact manner?”
“By inference from the habits of the parasite,” Thorndyke explained. “This particular filaria is distributed by the mosquito, and its habits are adapted to the habits of the mosquito. During the day, the worms are not found in the blood; they remain hidden in the tissues of the body. But about nine o’clock at night they begin to migrate from the tissues into the blood, and remain in the blood during the hours when the mosquitoes are active. Then about six o’clock in the morning, they leave the blood and migrate back into the tissues.
“There is another very similar species—Filaria diurna—which has exactly opposite habits, adapted to day-flying suctorial insects. It appears in the blood about eleven in the forenoon and goes back into the tissues at about six o’clock in the evening.”
“Astonishing!” exclaimed the coroner. “Wonderful! By the way, the parasites that you found could not, I suppose, have been Filaria diurna?”
“No,” Thorndyke replied. “The time excludes that possibility. The blood was certainly shed after six. They were undoubtedly nocturna, and the large numbers found suggest a late hour. The parasites come out of the tissues very gradually, and it is only about midnight that they appear in the blood in really large numbers.”
“That is very important,” said the coroner. “But does this disease affect any particular class of persons?”
“Yes,” Thorndyke replied. “As the disease is con fined to tropical countries, the sufferers are naturally residents of the tropics, and nearly always natives. In West Africa, for instance, it is common among the negroes but practically unknown among the white residents.”
“Should you say that there is a distinct probability that this unknown person was a negro?”
“Yes. But apart from the filaria there is direct evidence that he was. Searching for some cause of the bleeding, I noticed a lamp-hook screwed into the ceiling, and low enough to strike a tall man’s head. I examined it closely, and observed on it a dark, shiny mark, like a blood-smear, and one or two short coiled hairs which I recognised as the scalp-hairs of a negro. I have no doubt that the unknown man is a negro, and that
